1. Field of the Invention
The present invention relates to an improved process wherein high melt index ethylene homopolymer and low melt index ethylene copolymer having a narrowed short-chain branching distribution are prepared in separate polymerization zones to provide polyethylene resins having improved physical characteristics. Specific alkoxysilane modifiers are introduced into the copolymerization reactor to effect the desired modification of short-chain branching distribution.
2. Description of Prior Art
Two-step processes for polymerizing ethylene using a Ziegler-type catalyst dispersed in an inert hydrocarbon medium such as hexane or heptane are known. U.S. Pat. No. 4,357,448 describes such a process wherein polyethylene produced in a first polymerization zone is fed into a second polymerization zone together with the catalyst, solvent and unreacted ethylene. Additional ethylene and a comonomer are added to the second polymerization reactor and the final product is recovered. The polymerizations are carried out at relatively low temperatures and pressures using catalysts having high catalytic activity comprising an organoaluminum compound and a specific solid catalytic component obtained by reacting a titanium or vanadium halogen-containing compound with a reaction product obtained by reacting a Grignard reagent with a hydropolysiloxane of the formula
      R    a    ⁢      H    b    ⁢  S  ⁢          ⁢  i  ⁢          ⁢      O                  4        -        a        -        b            2      where R represents an alkyl, aryl, aralkyl, alkoxy, or aryloxy group, a is 0, 1 or 2; b is 1, 2 or 3; and a+b is ≦3 or a silicon compound containing an organic group and hydroxyl group in the presence or absence of an aluminum-alkoxide, aluminum alkoxy-halide or a reaction product obtained by reacting the aluminum compound with water.
Resins produced by these processes have good processability and extrudability making them highly useful for the manufacture of film, blow molded articles, pipe, etc.
The ability to control the molecular weight distribution of the final resin product in such processes using hydrogen as a molecular weight regulator and varying the ratio of the polymer produced in the first step to the polymer produced in the second step is highly advantageous and provides manufacturing versatility. It would be even more advantageous if in two-step processes of the above types where ethylene homopolymer is produced in the first step and ethylene copolymer is produced in the second step if the short-chain branching distribution of the copolymer produced in the second step could be narrowed. By narrowing the short-chain branching distribution resins having characteristics more closely resembling those of metallocene resins could be produced.
Various silane compounds have been utilized for the preparation of transition metal catalysts. U.S. Pat. No. 6,171,993, for example, discloses reacting various hydrocarbyl alkoxysilanes with the contact product of an organomagnesium compound and chemically treated support prior to contacting with the transition metal compound. Catalysts prepared in the above manner are combined with conventional aluminum-containing cocatalysts for the copolymerization of ethylene and hexene-1 or butene-1 in either gas phase or solution phase processes.
Various organosilicon compounds are also employed with Ziegler-Natta catalysts as selectivity control agents, primarily for the polymerization of propylene and ethylene to modify isotacticity. These processes are discussed in detail in PCT International Publication No. WO 2005/005489 A1. The process disclosed utilizes a specific mixture, preferably comprised of three silane compounds, for the polymerization of propylene or mixtures of propylene and ethylene.
U.S. Pat. No. 6,642,326 discloses the use of hydrosilane and polysiloxyhydrosilane modifiers to enhance the activity of boraaryl single-site catalysts.
The addition of tetraalkylorthosilicates to gas phase fluidized bed olefin polymerization reactors is disclosed in U.S. Pat. No. 5,731,392 to control static and thus reduce buildup of polymeric material on the reactor walls.